The invention relates to a rotating anode X-ray tube provided with a charge flow device, in which the anode current constituted by these charges flows without material contact between the fixed parts and the rotating parts of the tube, the latter being of the type having magnetic bearings. Such an X-ray tube can be used in all fields of radiology and particularly that of medical diagnosis.
Medical diagnosis X-ray tubes are generally constructed in the same way as a diode, i.e. comprising two electrodes, one of which called the cathode emits electrons, whilst the other electrode called the anode or anticathode receives these electrons over a small surface constituting the focus. These electrodes are enclosed in a vacuum-tight envelope and permit the electrical insulation between the two electrodes.
The cathode has a concentration part in which is located a heated filament constituting the electron source. When high voltage is applied to the terminals of the two electrodes in such a way that the cathode is at negative potential, a so-called anode current is established in the circuit across the high voltage generator. This current passes through the space between the cathode and the anode in the form of an electron beam, whose intensity is dependent on the temperature of the filament. This temperature is a function of the electric power dissipated in the filament. The anode current intensity is also dependent on the value of the high voltage applied between cathode and anode, but also on the power dissipated in the filament. For a given value of this high voltage, the quantity of X-rays emitted in the focus is proportional to the intensity of the anode current.
Most of the presently used diagnostic X-ray tubes comprise a rotating anode, which is rotated by means of mechanical bearings of the ball bearing type.
In such tubes, the only material contact between the rotating parts of the anode and the fixed parts of the pin connected to the positive polarity of the high voltage is brought about by the balls of these bearings. All the anode current passes through the contact points of these balls. It is known that mechanical bearing tubes have a shortened life, particularly due to the wear to the ball bearings. One of the causes of wear is the lubrication, which has to be a dry lubrication as a result of the vacuum in the tube.
An important improvement consists of installing the rotating anode with magnetic bearings. The latter generally comprise electromagnets arranged in pairs and in opposition, which produce magnetic fields under the influence of which a rotor, which is integral with the rotating anode whose rotation it ensures, is maintained in equilibrium. The rotating anode and the mechanical parts accompanying it in rotation are consequently not in mechanical contact with the remainder of the X-ray tube.
However, this causes a new problem, due to the fact that as the anode is mechanically insulated from the remainder of the tube, it is also electrically insulated therefrom. In addition, its high voltage connection and the flow of anode current make it possible to install means able to fulfil these functions in such tubes.
The solutions proposed are often friction piece or ball systems, i.e. mechanical and as a result part of the advantage of the magnetic bearing system is lost, namely the total absence of mechanical friction.
Another solution permitting the flow of anode current without mechanical contact uses the emission of electrons generated by thermo-emissive cathodes, mechanically connected to the rotating anode, said electrons being trapped by the fixed anode.
One of the main difficulties is then to supply the rotating thermo-emissive cathodes with the energy necessary for raising their temperature to an adequate level to comply with the thermoelectronic emission laws. Another difficulty encountered in this solution is the stabilization of the temperature of the thermo-emissive cathodes. As stated hereinbefore, the anode current of the tube is also dependent on the temperature of the electron source.